In one aspect, the invention relates to an apparatus suitable for forming a blow molded article. In another aspect, the invention relates to a method for blow molding a plastic article.
Blow molding processes are widely used to form hollow articles from plastic materials. Generally, a parison, which is a tube-shaped blank of molten plastic material is extruded from a nozzle having a generally annular opening defined between a die ring and land and is directed by gravity to between the parts of an opened mold. When a desired length of parison has been extruded, the mold is closed and compressed gas, usually air, is injected into the parison to pneumatically expand it to fit the inside contours of the mold.
When hollow articles having a cross section differing substantially from the circular are to be blow molded, the sides of the circular cross section parison are expanded to different degrees so there occurs a thinning of the plastic material as the parison is stretched. Localized weaknesses and flaws in blow molded articles thus sometimes occur where the parison has been called on to stretch too great an extent. This problem is especially severe where the blow molded article is to have sharp corners.
Another type of hollow article which is difficult to satisfactorily blow mold is one having inwardly directed indentions or folds. It is very difficult to achieve satisfactory wall distribution around the sides of the indention in the blow molded part. Blow molded parts having unacceptably thin spots in the walls or ruptures were commonplace. One attempt to alleviate this problem involves extruding a thicker walled parison, where the greater wall thickness may be localized if desired by shaping the die ring and/or land. While this approach provides some benefits processes for controlling wall thicknesses without using larger amounts of material would clearly be very desirable.